Cellulose is the most abundant organic matter on Earth. It is a renewable resource with no concern of depletion unlike petroleum or coal. However, cellulose is mostly discarded as agricultural and forestry wastes, which are regarded as the major causes of environmental pollution. Each year, more than 3 billion tons of agricultural and forestry wastes are produced worldwide, and more than 800 million tons in Asia only.
Since the agricultural and forestry wastes are mostly composed of cellulose and hemicellulose, if they can be converted to monosaccharides including glucose through saccharification, it may be greatly helpful for solving food, fuel and environmental problems.
As a general method of recovering monosaccharides from agricultural and forestry wastes, a method of adding sulfuric acid and conducting saccharification at high temperature and pressure is known. However, this method is problematic in that expensive equipment that can endure the strong acid and high pressure is necessary, separation and recovery of monosaccharides are difficult because of production of various byproducts, production cost is high because of the need to dispose of the byproducts and the associated processes are environment-unfriendly. Because the general saccharification method is limited for commercial application, researches have been made on a more environment-friendly cellulose saccharification method that can replace it. In this regard, various saccharification enzymes have been developed and commercialized for various industrial fields. Also, their applications are actively being studied. As the saccharification enzyme, cellulase is widely used in textile, papermaking, detergent and feed industries. In addition, it is used to produce low-calorie foods, ferment food wastes, or the like.
The cell wall of a plant consists of polymers such as cellulose (insoluble β-1,4-glucan fiber), hemicellulose (non-cellulose-based polysaccharide) and lignin (complex polyphenol polysaccharide). Among the components, cellulose is present in highest quantity, followed by hemicellulose with xylan as a main constituent. The two components account for more than 50% of the total plant biomass. Cellulose is a homopolymer of glucose units linked by β-1,4 linkage. To break it down into monosaccharides, three types of enzymes, i.e., endo-β-1,4-glucanase (endo-β-1,4-) [EC 3.2.1.4], exo-β-1,4-glucanase [EC 3.2.1.91] and β-glucosidase, are necessary. Endo-glucanase cleaves β-1,4 glucose linkages randomly from inside and exo-glucanase cleaves breaks down a glucan into the disaccharide cellobiose at a non-reducing end. Cellobiose is finally broken down to glucose by β-glucosidase.
Cellulase is mostly produced using molds (fungi). In particular, it is produced industrially using Aspergillus and Trichoderma. Although Trichoderma reesei ZU-02 (Trichoderma reesei ATCC 56764) has been intensively studied as a representative cellulase-producing strain, enzyme concentration and activity are not sufficient enough to satisfy the industrial needs. For example, although the process of producing ethanol from biomass has many advantages in terms of recycling of resources, environment-friendliness of the produced fuel, etc., the ethanol production from the lignocellulosic substance is highly costly as compared to gasoline production. In the ethanol production process, the cost of producing saccharification enzyme accounts for about 60% of the total production cost.
Accordingly, there is a strong need for an effective saccharification process of cellulose, particularly on environment-friendly one using a new strain.